Conventional feed-forward amplifiers can be generally placed into various categories identified as types I, II, and III, based on the way a desired signal and error signal are distributed through a various amplifiers and couplers comprising a feed-forward loop. A conventional type I feed-forward amplifier achieves a high degree of intermodulation (IM) reduction, typically about 20 dB, but has poor efficiency. Traditional type I feed-forward amplifiers only have an efficiency of about 5% which leads to high operating costs.
Conventional type II feed-forward amplifiers have improved efficiency compared to type I amplifiers, but type II amplifiers have a drawback of limited IM reduction. Also, conventional type II amplifiers reduce system gain, typically about 4 dB. Type III feed-forward amplifiers offer a compromise between efficiency and IM improvement (about 20 dB), but with further reduced system gain of about 7 dB. In addition, due to the complexity of type III feed-forward amplifiers, it is difficult to control these amplifiers to achieve optimum IM reduction.
Accordingly, there is a need for an improved amplifier circuit that provides improved efficiency, easier control, and IM reduction without undesirably reducing system gain. Further, there is a need for an improved method of controlling feed-forward amplifiers.